A ratio of He ii \(\) 1640/\(\) 4686 can be found by comparing difference spectra between high and low states of He ii emission rather than from spectra themselves. Because the lower-ionization lines blending with He ii will not vary as much as He ii, difference spectra between high and low states will mostly show the highly variable He ii line. The variable component of the He ii lines can thus readily be identified by subtracting a low state from a high state for which simultaneous optical and UV spectra are available. During the 1993 International AGN Watch monitoring campaign [1]} simultaneous (\(\pm 1\) d) HST and optical spectra were obtained on five days. Figure 2 shows continuum-subtracted UV and optical difference spectra for the regions around the He ii lines for JD 2449128 and JD 2449099. It can be see that this has removed all of the O iii] and broad optical Fe ii emission and that the difference profiles for the two He ii lines agree well. The observed ratio is 4. For an intrinsic ratio of 8.5 this gives \(E(1640-4686) = 0.82\)  magnitudes
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